Omega-3 highly unsaturated fatty acids are of significant commercial interest in that they have been recently recognized as important dietary compounds for preventing arteriosclerosis and coronary heart disease, for alleviating inflammatory conditions and for retarding the growth of tumor cells. These beneficial effects are a result both of omega-3 highly unsaturated fatty acids causing competitive inhibition of compounds produced from omega-6 fatty acids, and from beneficial compounds produced directly from the omega-3 highly unsaturated fatty acids themselves (Simopoulos et al., 1986). Omega-6 fatty acids are the predominant highly unsaturated fatty acids found in plants and animals. Currently the only commercially available dietary source of omega-3 highly unsaturated fatty acids is from certain fish oils which can contain up to 20-30% of these fatty acids. The beneficial effects of these fatty acids can be obtained by eating fish several times a week or by daily intake of concentrated fish oil. Consequently large quantities of fish oil are processed and encapsulated each year for sale as a dietary supplement.
However, there are several significant problems with these fish oil supplements. First, they can contain high levels of fat-soluble vitamins that are found naturally in fish oils. When ingested, these vitamins are stored and metabolized in fat in the human body rather than excreted in urine. High doses of these vitamins can be unsafe, leading to kidney problems or blindness and several U.S. medical associations have cautioned against using capsule supplements rather than real fish. Secondly, fish oils contain up to 80% of saturated and omega-6 fatty acids, both of which can have deleterious health effects. Additionally, fish oils have a strong fishy taste and odor, and as such cannot be added to processed foods as a food additive, without negatively affecting the taste of the food product. Moreover, the isolation of pure omega-3 highly unsaturated fatty acids from this mixture is an involved and expensive process resulting in very high prices ($200-$1000/g) for pure forms of these fatty acids (Sigma Chemical Co., 1988; CalBiochem Co. , 1987).
The natural source of omega-3 highly unsaturated fatty acids in fish oil is algae. These highly unsaturated fatty acids are important components of photosynthetic membranes. Omega-3 highly unsaturated fatty acids accumulate in the food chain and are eventually incorporated in fish oils. Bacteria and yeast are not able to synthesize omega-3 highly unsaturated fatty acids and only a few fungi are known which can produce minor and trace amounts of omega-3 highly unsaturated fatty acids (Weete, 1980; Wassef, 1977; Erwin, 1973).
Algae have been grown in outdoor cultivation ponds for the photoautotrophic production of a wide variety of products including omega-3 highly unsaturated fatty acid containing biomass. For example, U.S. Pat. No. 4,341,038 describes a method for the photosynthetic production of oils from algae, and U.S. Pat. No. 4,615,839 describes a process for concentrating eicosapentaenoic acid (EPA) (one of the omega-3 highly unsaturated fatty acids) produced photosynthetically by strains of the green alga Chlorella. Photoautotrophy is the process whereby cells utilize the process of photosynthesis to construct organic compounds from CO.sub.2 and water, while using light as an energy source. Since sunlight is the driving force for this type of production system, algal cultivation ponds require large amounts of surface area (land) to be economically viable. Due to their large size, these systems cannot be economically covered, because of high costs and technical problems, and because even transparent covers tend to block a significant amount of the sunlight. Therefore, these production systems are not axenic, and are difficult to maintain as monocultures. This is especially critical if the cultures need to be manipulated or stressed (e.g. nitrogen limited) to induce production of the desired product. Typically, it is during these periods of stress, when the cells are only producing product and are not multiplying, that contaminants can readily invade the cultures. Thus, in most cases, the biomass produced is not desirable as a food additive for human consumption without employing expensive extraction procedures to recover the lipids. Additionally, photosynthetic production of algae in outdoor systems is very costly, since cultures must be maintained at low densities (1-2 g/l) to prevent light limitation of the culture. Consequently, large volumes of water must be processed to recover small quantities of algae, and since the algal cells are very tiny, expensive harvesting processes must also be employed.
Mixotrophy is an alternative mode of production whereby certain strains of algae carry on photosynthesis with light as a necessary energy source but additionally use organic compounds supplied in the medium. Higher densities can be achieved by mixotrophic production and the cultures can be maintained in closed reactors for axenic production. U.S. Pat. Nos. 3,444,647 and 3,316,674 describe processes for the mixotrophic production of algae. However, because of the need to supply light to the culture, production reactors of this type are very expensive to build and operate, and culture densities are still very limited.
An additional problem with the cultivation of algae for omega-3 highly unsaturated fatty acid production, is that even though omega-3 highly unsaturated fatty acids comprise 20-40% of some strains' total fatty acids, the total fatty acid content of these algae is generally very low, ranging from 5-10% of ash-free dry weight. In order to increase the fatty acid content of the cells, they must undergo a period of nitrogen limitation which stimulates the production of lipids. However, of all the strains noted to date in the literature, and over 60 strains evaluated by the inventor, all exhibit a marked decrease in omega-3 highly unsaturated fatty acids as a percentage of total fatty acids, when undergoing nitrogen limitation (Erwin, 1973; Pohl & Zurheide, 1979).
With respect to economics and to utilizing omega-3 highly unsaturated fatty acids as a food additive, it would be desirable to produce these fatty acids in a heterotrophic culture. Heterotrophy is the capacity for sustained and continuous growth and cell division in the dark in which both energy and cell carbon are obtained solely from the metabolism of an organic substrate(s). Since light does not need to be supplied to a heterotrophic culture, the cultures can be grown at very high densities in closed reactors. Heterotrophic organisms are those which obtain energy and cell carbon from organic substrates, and are able to grow in the dark. Heterotrophic conditions are those conditions that permit the growth of heterotrophic organisms, whether light is present or not. However, the vast majority of algae are predominantly photoautotrophic, and only a few types of heterotrophic algae are known. U.S. Pat. Nos. 3,142,135 and 3,882,635 describe processes for the heterotrophic production of protein and pigments from algae such as Chlorella, Spongiococcum, and Prototheca. However these genera and others that have been documented to grow very well heterotrophically (e.g. Scenedesmus), do not produce omega-3 highly unsaturated fatty acids (Erwin, 1973). The very few heterotrophic algae known to produce any omega-3 highly unsaturated fatty acids (e.g., apochlorotic diatoms or apochlorotic dinoflagellates) generally grow slowly and produce low amounts of omega-3 highly unsaturated fatty acids as a percentage of ash-free dry weight (Harrington and Holtz, 1968; Tornabene et al., 1974).
A few higher fungi are known to produce omega-3 highly unsaturated fatty acids, but they comprise only a very small fraction of the total fatty acids in the cells (Erwin, 1973; Wassef, 1977; Weete, 1980). As such, they would not be good candidates for commercial production of omega-3 highly unsaturated fatty acids. For example, Yamada et al. (1987) recently reported on the cultivation of several species of the fungus, Mortierella, (isolated from soils) for the production of the omega-6 fatty acid, arachidonic acid. These fungi also produce small amounts of omega-3 eicosapentaenoic acid along with the arachidonic acid when grown at low temperatures (5.degree.-24.degree. C.). However, the resulting eicosapentaenoic acid content was only 2.6% of the dry weight of the cells, and the low temperatures necessary to stimulate production of this fatty acid in these species would result in greatly decreased productivities (and economic potential) of the cultivation system. Some single-celled members of the order Thraustochytriales are also known to produce omega-3 highly unsaturated fatty acids (Ellenbogen, 1969, Wassef, 1977; Weete, 1980; Findlay et al., 1986) but they are known to be difficult to culture. Sparrow (1960) noted that the minuteness and simple nature of the thalli of the family Thraustochytriaceae (order Thraustochytriales) make them exceedingly difficult to propagate. Additional reasons for this difficulty have been outlined by Emerson (1950) and summarized by Schneider (1976): "1) these fungi consist of very small thalli of only one or a few cells, which generally grow very slowly in culture, and are very sensitive to environmental perturbation; 2) they are generally saprophytes, or parasites with very specialized nutritional and environmental demands; and 3 ) in pure culture they generally exhibit restricted growth, with vegetative growth terminating after a few generations." (Although some prior art classifies the thraustochytrids as fungi, the most recent consensus is that they should be classified as algae, see discussion below.)
As a result little attention has been paid to the numerous orders of these microorganisms, and those studies that have been conducted, have been predominantly carried out with a taxonomic or ecological focus. For example, even though the simple fatty acid distribution of several members of the order Thraustochytriales has been reported from a taxonomic perspective (Ellenbogen, 1969); Findlay et al., 1986), no one has ever reported their total fatty acid content or lipid content as percent dry weight. Unless data on the total lipid content is available, one cannot evaluate an organism's potential for use in the production of any type of fatty acid. For example, the omega-3 highly unsaturated fatty acid content of the lipids of some marine macroalgae (seaweeds) is reported to be very high, up to 51% of total fatty acids (Pohl & Zurheide, 1979). However, the lipid content of macroalgae is typically very low, only 1-2% of cellular dry weight (Ryther, 1983). Therefore, despite the reported high content of omega-3 highly unsaturated fatty acids in the fatty acids of macroalgae, they would be considered to be very poor candidate organisms for the production of omega-3 highly unsaturated fatty acids. Despite a diligent search by the inventor, no reports of simple proximate analysis (% protein, carbohydrate and lipid) of the Thraustochytriales has been found, nor has anyone reported attempts to cultivate these species for purposes other than laboratory studies of their taxonomy, physiology or ecology. Additionally, many of the strains of these microorganisms have been isolated by simple pollen baiting techniques (e.g., Gaertner, 1968). Pollen baiting techniques are very specific for members of the Thraustochytriales, but do not select for any characteristics which may be desirable for large scale cultivation of microorganisms.
Thus, until the present invention, there have been no known heterotrophic organisms suitable for culture that produce practical levels of omega-3 highly unsaturated fatty acids and such organisms have been thought to be very rare in the natural environment.